Mathematics for the interested outsider

It turns out that our existence proof will actually hinge on our function satisfying a Lipschitz condition. So let’s show that we will have this property anyway.

More specifically, we are given a function defined on an open region . We want to show that around any point we have some neighborhood where satisfies a Lipschitz condition. That is: for and in the neighborhood , there is a constant and we have the inequality

We don’t have to use the same for each neighborhood, but every point should have a neighborhood with some .

Infinitesimally, this is obvious. The differential is a linear transformation. Since it goes between finite-dimensional vector spaces it’s bounded, which means we have an inequality

where is the operator norm of . What this lemma says is that if the function is we can make this work out over finite distances, not just for infinitesimal displacements.

So, given our point let be the closed ball of radius around , and choose so small that is contained within . Since the function — which takes points to the space of linear operators — is continuous by our assumption, the function is continuous as well. The extreme value theorem tells us that since is compact this continuous function must attain a maximum, which we call .

The ball is also “convex”, meaning that given points and in the ball the whole segment for is contained within the ball. We define a function and use the chain rule to calculate

Then we calculate

And from this we conclude

That is, the separation between the outputs is expressible as an integral, the integrand of which is bounded by our infinitesimal result above. Integrating up we get the bound we seek.

I have to take a little detour for now to prove an important result: the existence and uniqueness theorem of ordinary differential equations. This is one of those hard analytic nubs that differential geometry takes as a building block, but it still needs to be proven once before we can get back away from this analysis.

Anyway, we consider a continuously differentiable function defined on an open region , and the initial value problem:

for some fixed initial value . I say that there is a unique solution to this problem, in the sense that there is some interval and a unique function satisfying both conditions.

In fact, more is true: the solution varies continuously with the starting point. That is, there is an interval around , some neighborhood of and a continuously differentiable function called the “flow” of the system defined by the differential equation , which satisfies the two conditions:

Then for any we can get a curve defined by . The two conditions on the flow then tell us that is a solution of the initial value problem with initial value .

This will take us a short while, but then we can put it behind us and get back to differential geometry. Incidentally, the approach I will use generally follows that of Hirsch and Smale.

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This is mainly an expository blath, with occasional high-level excursions, humorous observations, rants, and musings. The main-line exposition should be accessible to the “Generally Interested Lay Audience”, as long as you trace the links back towards the basics. Check the sidebar for specific topics (under “Categories”).

I’m in the process of tweaking some aspects of the site to make it easier to refer back to older topics, so try to make the best of it for now.